This study was undertaken to test the hypothesis that children with autistic spectrum disorders often have macrocephalus, and that those without comorbid learning disability are most frequently affected. Fifty consecutive children with Asperger syndrome (45 males, five females; mean age 9 years, range 1 year 6 months to 16 years) without indications of underlying medical disorders were matched for birth year and sex with 50 children (45 males, five females; mean age 6 years 4 months, range 1 year 4 months to 13 years 11 months) who met criteria for autistic disorder (a lower-functioning disorder within the autism spectrum) and with 50 children (45 males, five females; mean age 8 years 4 months, range 1 year 6 months to 15 years 5 months) who met criteria for attention-deficit–hyperactivity disorder. Birth and neuropsychiatric follow-up records were examined and data relating to occipitofrontal circumference, weight, and height were detailed. The group with Asperger syndrome included a subset of individuals with macrocephalus recorded both at birth and at follow-up after the first year of life. Another subgroup developed macrocephalus during early childhood. Autistic spectrum disorders include a subgroup with macrocephalus characterized by a relatively high level of functioning and a clinical presentation most often consistent with a diagnosis of Asperger syndrome.